[libc-commits] [libc] [wip] [libc] Implement vasprintf (PR #98824)

[Joseph Huber via libc-commits]

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@@ -0,0 +1,81 @@
+//===-- Implementation of vasprintf -----------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/stdio/vasprintf.h"
+
+#include "src/__support/OSUtil/io.h"
+#include "src/__support/arg_list.h"
+#include "src/stdio/printf.h"
+#include "src/stdio/printf_core/core_structs.h"
+#include "src/stdio/printf_core/printf_main.h"
+#include "src/stdio/printf_core/writer.h"
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h> // malloc, realloc, free
+
+namespace LIBC_NAMESPACE {
+namespace {
+
+LIBC_INLINE int resize_overflow_hook(cpp::string_view new_str, void *target) {
+  printf_core::WriteBuffer *wb =
+      reinterpret_cast<printf_core::WriteBuffer *>(target);
+  size_t new_size = new_str.size() + wb->buff_cur;
+  char *TmpBuf =
+      static_cast<char *>(realloc(wb->buff, new_size + 1)); // +1 for null
+  if (TmpBuf == nullptr) {
+    return -1;
+  }
+  wb->buff = TmpBuf;
+  inline_memcpy(wb->buff + wb->buff_cur, new_str.data(), new_str.size());
+  wb->buff_cur = new_size;
+  wb->buff_len = new_size;
+  return printf_core::WRITE_OK;
+}
+
+} // namespace
+
+inline constexpr size_t DEFAULT_BUFFER_SIZE = 200;
+
+LLVM_LIBC_FUNCTION(int, vasprintf,
+                   (char **__restrict ret, const char *format, va_list vlist)) {
+  internal::ArgList args(vlist); // This holder class allows for easier copying
+                                 // and pointer semantics, as well as handling
+                                 // destruction automatically.
+  auto init_buff = static_cast<char *>(malloc(DEFAULT_BUFFER_SIZE));
----------------
jhuber6 wrote:

We probably want to avoid the `malloc` until the end. It's cheaper to work with a few bytes on the stack and then `memcpy` it, you can use the `libc` memcpy routines. That way `vasprintf(buffer, "%s", "");` doesn't use 200 bytes instead of 1.

https://github.com/llvm/llvm-project/pull/98824